The present invention relates to zirconia base ceramics.
Zirconia base ceramics (hereinafter referred often to as ZrO.sub.2 ceramics) generally has high toughness, but involves an essential problem of deterioration with the lapse of time due to phase transition.
During cooling, ZrO.sub.2 undergoes a martensitic-type transformation from a tetragonal crystal structure to a monoclinic crystal structure with a concurrent increase in volume and an anisotropic shape change. For pure ZrO.sub.2 the transformation begins at about 1200.degree. C. and proceeds until complete at about 600.degree. C.
Attempts have been made to utilize this transformation in order to improve the fracture toughness of ceramic composites.
For that reason, the addition of stabilizers is usually carried out for stabilization or partial stabilization, as disclosed in a number of literatures (U.S. Pat. No. 4,316,964; Japanese Patent Kokoku-Publication No. 54-25523). Attempts have also been made to improve the properties of ZrO.sub.2 ceramics by the selective use of a specific stabilizer. First of all, Japanese Patent Kokai-Publication No. 56-134564 discloses that Y.sub.2 O.sub.3 is selected for the purpose of suppressing the deterioration with time at a certain temperature, T. K. Gupta et al, Journal of Materials Science, 12 (1977) teaches that the same compound is used to improve strength. On the other hand, Japanese Patent Kokai-Publication Nos. 59-152266 and 59-190265 teach the selective addition of CeO.sub.2 for the purpose of improving thermal shock resistance.
Other attempts have been made to achieve the improvements in properties from another point-of-view by the addition of a third component in addition to the stabilizers. For instance, Japanese Patent Kokai-Publication No. 58-32066 discloses that further improvements in strength are intended by the selective addition of Al.sub.2 O.sub.3, and Japanese Patent Kokai-Publication No. 58-172265 describes that the low coefficient of expansion is obtained by the same means.
As mentioned above, the zirconia base ceramics have hitherto received special attention due to its high strength and high resistance to fracture. This high strength and high resistance to fracture of these ceramics have been thought to be attributable to the stress induced phase transformation. However, it has been shown that the high strength and high resistance of zirconia base ceramics, especially Y.sub.2 O.sub.3 -PSZ, was greatly decreased by low-temperature annealing such as in the range 200.degree. C. to 400.degree. C. in air. The loss of strength and fracture toughness by annealing is due to the formation of microcracking accompanied by the tetragonal to monoclinic phase transformation on the surface of the sintered materials. This degradation accompanied by the tetragonal to monoclinic phase transformation of Y.sub.2 O.sub.3 -PSZ by low temperature annealing in wet atmosphere occurs with a high rate and at lower temperatures than the case in a dry atmosphere. Namely, yttria stabilized zirconias are not stable at low temperatures (around 300.degree. C.) in the presence of steam.
However, until now there have not been any zirconia base ceramics that are high in both toughness and strength and have satisfactory thermal and hydrothermal stabilities.